The development of open tubular columns, particularly those having very narrow bores of capillary dimensions, has been very significant, particularly in gas chromatography.
A major shortcoming of wall-coated open tubular columns (WCOT) used for gas chromatography is their relatively high phase ratio, i.e., the ratio of the volume of mobile phase per unit column length to the volume of stationary phase per unit column length. This ratio is generally denoted by the symbol .beta., and usually has a numerical value between 150 and 500. This value of .beta. leads to low values of partition ratio, which in turn decreases the number of effective theoretical plates. Increasing the thickness of the layer of stationary phase on the column wall in attempting to improve the values of .beta. increases resistance to mass transfer with a resultant decrease in the number of effective theoretical plates per unit column length.
Closely related to the problem of high phase ratio is the limited sample capacity of open tube columns due to the small amount of stationary retarding phase per unit length of column. As a result, the available efficiency of such columns cannot always be fully utilised for analytical purposes owing to limited sensitivity of conventional detectors.
Attempts to improve the phase ratio of capillary columns have included roughening the inner walls of the column by etching and by depositing sodium chloride thereon. These methods can produce only a limited increase in surface area of the column walls and, therefore, only a limited improvement in .beta.-values.
Acceptable .beta.-values have been obtained by making support-coated open tubular columns (SCOT) in which a layer of support particles, e.g. diatomaceous earth, is deposited on the inner walls of the tube. However, it is very difficult to produce SCOT columns with uniform and completely reproducible characteristics. SCOT columns also have a comparatively short useful life. A feature of capillary open tubular columns is that their separating efficiency in terms of .beta.-values and plate height improves as the diameter decreases. However, SCOT columns with diameters below about 0.5 mm cannot be made by conventional methods.
Accordingly, there is a continuing demand for improved open-tubular chromatography columns.